Hitherto, solar modules often have not been equipped with special safety provisions. The solar connection box (also known as solar junction box) provided on each solar module is essentially used to connect the conductor strips (also referred to as ribbons) of the solar module and to accommodate the bypass diodes. Such simple solar modules will provide voltage at the output contacts of the solar junction box as soon as they are irradiated.
Typically, the solar modules are series-connected so as to form strings in order to obtain a high string voltage. This voltage of the solar modules connected into strings can amount to several 100 V, and in large systems operation voltages of about 1000 V or even more may exist, which is a hazardous voltage when contacted. Also, a solar power plant may optionally comprise a plurality of strings connected in parallel.
Thus, in case of adequate irradiation, very high voltages might be generated during the installation of the solar modules and electrical wiring thereof in the string, so that cautious handling and a special contact protection is necessary. The same applies to maintenance works and incidents, such as in case of a fire.
In such simple photovoltaic systems, the first possibility for disconnecting the solar generator voltage from the power grid is at the generator junction box. Therefore, in case of damage caused by fire, water, hail, or in case of other malfunctions of the solar modules or the string lines, there is no possibility in these simple solar modules to de-energize or disconnect from voltage the part of the photovoltaic system that includes the solar modules and the string lines.
Safety devices for switching off the individual solar modules in cases of fire or faults are known (e.g. DE 10 2009 024 516 A1). However, such solutions are usually only intended for a one-time emergency shutdown.
From EP 2 726 889 B1, a photovoltaic module is known which may create a safe shutdown in the case a module has not been installed or connected, or in case of a fault. For this purpose, a unit with a test device is integrated either directly in the junction box of the photovoltaic module or locally associated with a photovoltaic module. In order to switch on the photovoltaic module, the test device of the unit checks whether a fault state still exists or whether the individual photovoltaic module is connected to an inverter. In any case, the shutdown of the photovoltaic module takes place locally on the photovoltaic module itself, so that the unit with the test device is arranged locally on the photovoltaic module in any case. However, to provide the test device locally on the individual photovoltaic module is complicated and therefore disadvantageous. Furthermore, central control of a string that comprises a plurality of photovoltaic modules is in principle not possible in this way. Moreover, surge currents or impulse currents might flow in this case, which may adversely affect the service life of the components involved in current flow. Furthermore, the input capacitor of the inverter might be relatively large, which may result in the source being unable to deliver enough power at startup, so that the charging of the input capacitor is only possible to a limited extent.
DE 10 2011 110 682 A1 describes a junction box for a solar panel with a protective circuit, which is in a protective state when being delivered and switches into the protective state in case of an incident. In the protective state, the solar cells of the solar panel are disconnected from the string. The protective circuit may have external terminals, such as those of the junction box, are short-circuited in the protective state. In this manner a voltage is reliably prevented from being applied to the external terminals of, e.g., the junction box in the protective state, and despite of that, the electrical circuit of the string is closed in the protective state, so that other solar panels can possibly continue to feed solar power. Furthermore, this also allows to centrally check and, if necessary, control the entire string.
Furthermore, from WO 2014/122325 A1 a safe photovoltaic system is known which comprises a central start box that starts an entire string comprising a plurality of series-connected solar modules each one in particular including a protective or safety circuit, either upon initial connection or upon reconnection. Since a dangerous contact voltage of, e.g., up to 1000 volts or more, might be applied at the string lines of such a string comprising a plurality of solar modules, the start box checks whether the circuit via the string lines between the start box and the entire string of the plurality of solar modules, i.e. between the start box and all safety circuits of the solar modules, is closed, in order to prevent a dangerous open contact voltage from being applied at possibly open terminals between the start box and the plurality of safety circuits. The section between the central start box and the inverter is not checked here. This is the starting point of the present disclosure which in particular is a further development of a photovoltaic system that comprises solar modules each of which have a safety circuit and a start box according to WO 2014/122325 A1, the subject matter of which is therefore incorporated herein by reference.